Developmental control of the heat shock response in Xenopus.

Xenopus cells express two major proteins on heat shock, designated hsp 70 and hsp 30. Several cDNA clones for the corresponding mRNAs were identified and sequenced. Inducibility and abundance of heat shock mRNAs in various cell types and developmental stages was determined by nuclease S1-mapping. The only cells found to contain hsp 70 mRNA without heat shock are the oocytes. The level of this stored hsp 70 mRNA is not increased by heat shock. After fertilization, hsp 70 mRNA becomes undetectable; it appears as a heat-inducible mRNA for the first time at gastrulation. After this stage, all somatic cell types accumulate hsp 70 mRNA to similar levels on heat shock, presumably by transcriptional activation of the hsp 70 genes. In contrast, hsp 30 mRNA is not detectable, even after heat shock, in oocytes or embryos that induce hsp 70 mRNA to high levels. Heat inducibility appears late in development--at the tadpole stage. However, the level of induced mRNA varies considerably in different adult tissues. This indicates that the Xenopus heat shock genes are not coordinately controlled. A long-term developmental control appears to be superimposed on the temporary heat inducibility of the heat shock genes: stage- or cell-type-specific conditions can lead to constitutive or repressed heat shock genes.